A car travels $\displaystyle \dfrac { 1 }{ 3 } $rd distance on a straight 3 road with a velocity of 10 km/hr, next $\displaystyle \dfrac { 1 }{ 3 } $rd with velocity $20 km/hr$ and the last $\displaystyle \dfrac { 1 }{ 3 } $rd with velocity $60 km/hr$. What is the average velocity of the car in the whole journey?